Somoth Ergai said:
How else do we even know how far away the planet is?
How do we know how far away the Andromeda galaxy is? Do we know that
just by looking at the light coming from that galaxy?
The answer is no, we don't. We have to
calculate how far away it is by combining the information in the light we see from it with other information from other sources. In the case of the Andromeda galaxy, we have to measure the periods of Cepheid variable stars we see in that galaxy, and then use a relationship for Cepheid variables between the period and the luminosity, which we have to obtain by observing Cepheid variable stars in our own galaxy, to calculate the luminosity of the Cepheids we see in the Andromeda galaxy. Then we combine that with the apparent brightness of those Cepheids to calculate their distance.
This is the sort of thing I was referring to when I said the light we see, by itself, doesn't tell us distances (or times). So we can't just "track the distance" by looking at the light. We have to already
know the distances some other way and then
calculate things.
Somoth Ergai said:
we can determine how far away the back ground stars are
What "background stars" are you talking about? The ship is coming straight from Andromeda to us. The only "background" we are seeing it against is the Andromeda galaxy itself. We certainly don't have "background stars" placed at convenient intervals all along the way that send us a signal when the ship passes them.
I don't think you have thought through very carefully what we would actually be observing in the scenario you have described.
Somoth Ergai said:
a photon crossing that distance (2 million light years) would take two million years from point A (their planet) to point B (Earth)
In the Earth's rest frame, yes. But note that this statement is
only true in the Earth's rest frame.
Somoth Ergai said:
the time it takes the ship to cross that distance, again from our perspective, is less than the time it would have taken a photon to cross that distance
But it
isn't. You have already been told this, repeatedly. But let me restate it again.
Call the time in the Earth's frame when the ship leaves Andromeda time zero. A light signal is emitted from the ship towards Earth at the same instant.
That light signal arrives at Earth at time 2 million years.
The ship itself arrives at Earth at time 2 million years plus about 10.5 minutes (per the calculations done in earlier posts).
In other words, the ship arrives
after the light. So it can't possibly take less time than the light takes to travel the same distance. If it did, the ship would arrive before the light, not after it.
Now let's talk about the "speed much faster than light" calculation. How is that calculated? It's calculated by taking the distance of 2 million years and dividing it by 10.5 minutes, i.e., the time elapsed on Earth between the light arriving and the ship arriving. But this same calculation would say that the light itself travels with
infinite speed, since the time between the light arriving and the light arriving is, well, zero. So here again the ship does not travel faster than the light does. It travels faster than ##c## by this calculation, but the light travels faster than ##c## too--in fact infinitely faster. This is why I say that describing this as "the ship appearing to travel much faster than light" is misleading--because you would then have to describe the light itself as traveling
infinitely faster than light.